This invention relates generally to vibration control in data storage devices, and more particularly to tracking control under vibration conditions in such devices. Data storage devices are provided in which tracking control is adapted to counter effects of vibration.
In data storage devices such as tape drives and disk drives, an actuator system effects movement of the read/write head relative to a data track of the storage medium to align the head with the data track. A servo control system uses servo information read from the storage medium to control the actuator system so as to maintain head/track alignment during operation. In tape drives, for example, the servo control system typically corrects transversal position of the head relative to longitudinal data tracks on tape as well as rotational positioning to counter skew between the head and tape. Reliable operation of such storage devices requires robust performance of the servo control system under vibration conditions. Standard vibration profiles are typically used to describe the vibration specifications in terms of the acceleration input under which the storage device must continue to operate reliably. Demand for increased storage density makes it increasingly more challenging to meet the performance specifications under vibration conditions. In tape drives for example, increasing the tape track density tightens further the tolerance in the acceptable track-following error during read/write operations, so improved track-follow performance is needed to keep the additional track-follow margin required for reliable tape operation under vibration conditions at a minimum.
There are currently two main techniques for improving track-follow performance in vibration environments. A first technique employed in tape drives involves switching between two track-follow controllers. A high-bandwidth controller performs better during vibrations but is less reliable during normal operation. Therefore, upon detection of vibration, the tape drive switches from a low-bandwidth controller to the higher bandwidth controller. The main drawbacks of this scheme are that it relies on reliable detection of the onset of a vibration condition, and there is then a transient behavior during switching between the two controllers. It is also difficult to evaluate stability and performance because of the hybrid control scheme.
A second technique for dealing with vibration conditions uses accelerometers to measure the applied vibrations. Some implementations of this technique use an accelerometer mounted on the head actuator, e.g. as in the disk drives described in U.S. Pat. Nos. 6,407,876 and 5,426,545. Other implementations of this technique employ accelerometers mounted on the drive body rather than the actuator. Examples are described in U.S. Pat. No. 7,468,857 and “Increased disturbance rejection in magnetic disk drives by acceleration feedforward control and parameter adaptation”, White and Tomizuka, Control Eng. Practice, Vol. 5, No. 6, pp. 741-751, 1997. Here the accelerometer information is utilized in a feedforward controller to correct the position of the actuator.